The correct adjustment of the air-pressure of vehicle wheels is, to begin with, of an economical importance because an incorrect adjustment, that is, an air-pressure which is too high or too low, leads to an increased wear in the tire whereby the wheels of the vehicle must be replaced prematurely. This causes, in particular in the case of lorries whose tires are normally very expensive, unnecessary costs. A tire pressure which is too low also causes an increased consumption.
However, of more importance than the economical aspect is the safety aspect. A faulty air-pressure in a vehicle wheel, in particular, air-pressure which is too low, causes an increased wearing-away of the rims of the tire, whereby the temperature of the tire is greatly increased and the strength of the tire rim is reduced. Due to this, the tire can suddenly be destroyed. Because the increased wear occurs, in particular, when the speed is high, such tire damage often leads to severe traffic accidents.
In order to avoid the economical disadvantages and, in particular, the danger of accidents, the air-pressure must be regularly checked, which by lorries means daily. However, the checking does not happen often because the tire pressure measurement is a relatively lengthy and also messy task which demands as well a certain technical skill.
In the patent literature, different suggestions have therefore been made to measure the tire air-pressure by way of pressure sensors arranged on the vehicle wheels, which measurement signal is then displayed in a suitable way to the driver. Such a suggestion is to be found, for example, in DE-3930479 A1.
However, the realisation of such a monitoring device in practice hits against considerable difficulties.
Since the vehicle wheel rotates during travel and a mechanical transmission of the measurement signals from the rotating wheel onto the non-rotating parts of the vehicle is normally not possible due to lack of space, the transmission of the measurement signals must be carried out by way of a wireless transmission. Therefore, apart from infrared transmission and ultrasonic transmission, above all, electromagnetic signal transmission is offered. However, the electromagnetic signal transmission comes with problems since, in a vehicle a number of sources of electrical signals are already at hand, for example, the ignition system, the light mechanism, electrical operated blower, as well as other electrical assisted motors, etc. Apart from this, there are more than enough external sources of disturbance, for example, tramways, signal crossings, also radio senders and such like, which can influence the transmission.
Concerning the reliability of a monitoring device, high demands must be made. By a disturbance, if the monitoring device is then not in a position to reliably indicate the occurrence of a result by the monitor, then such a device cannot meet it's contemplated purpose. However, on the other hand, should the monitoring device give off constant false alarms, then the driver will not consider the result any more and the system will then remain non-effective even when monitoring events have really occurred and are indicated.
Furthermore, in view of the necessary reliability, it is also to be considered that because such a monitoring device is present, a manual test of the tire pressure does not take place any more because the drivers respectively go on the assumption that a false tire-pressure adjustment will be indicated by the monitoring device.
The known monitoring devices in the art cannot fulfil these high requirements for reliability.
Hence, the task of the present invention is to provide a monitoring device as described above, which will allow a reliable measurement and indication of the air-pressure, and respectively, the air-pressure change in the air-chamber of a pneumatic tire of a vehicle wheel.